ENZYME entry: EC 4.1.99.13

The overall repair reaction consists of two distinct steps, one of
which is light-independent and the other one light-dependent.

In the initial light-independent step, a 6-iminium ion is thought to
be generated via proton transfer induced by two histidines highly
conserved among the (6-4) photolyases.

This intermediate spontaneously rearranges to form an oxetane
intermediate by intramolecular nucleophilic attack.

In the subsequent light-driven reaction, one electron is believed to
be transferred from the fully reduced FAD cofactor (FADH(-)) to the
oxetane intermediate thus forming a neutral FADH radical and an
anionic oxetane radical, which spontaneously fractures.

The excess electron is then back-transferred to the flavin radical
restoring the fully reduced flavin cofactor and a pair of pyrimidine
bases.